ItÂ´s common here in Brazil administrators setup the OS (windows or linux) to not observe DST but some companies observe DST.
On the Notes Client "The "Use the Operating System's Timezone Settings" should always be set to Yes for all Notes client versions
Setting this parameter to "No" may cause unpredictable results in currently supported versions of Notes software.
This setting is considered obsolete and is not recommended in any current version of IBM Notes software. Notes is designed to use the time and time zone settings provided by the client operating system (including the installed JRE), regardless of which operating system is in use.
A corresponding ini setting "UseNotesTimeZone" is not supported for Notes 6.0 and later.
Using either of these settings may cause errors in the Notes client.
Best Practice: Remove these settings from user's clients before troubleshooting time or time zone issues. The Notes client relies on the Operating system Time settings, and the use of these obsolete settings may cause errors under certain circumstances. "